From the very beginning I wanted to make a flythrough of this city, if not a more intensive animation. This meant I would be designing an actual enviroment, and not just a single scene shot from one camera. Thus, there was a lot more emphasis on filling the city itself with lots of interesting things to look at and go through during the animation.
Because it is a rather complex model, Ill touch on each general area only briefly.

The landform was created with a patch grid and a displace map. To keep that excitement of a air chase I stuck with tall cliff walls that surrounded the city on every side, except for some valleys that I figured on adding some volumetrics and making more interesting at a later time.

For the main city buildings, I designed a simple 'street map' on another patch grid with the faces representing the buildings and leaving room for the highway system between them.
I then added Greeble to the patch grid to make the actual buildings. Since this thing was bound to get complex later, I had to make the buildings relatively simplistic or else Id never be able to animate it in a timely fashion.

When it was obvious the city was going to be inside this hole in the ground, I figured that some sort of overhead structure may be useful, for providing additional housing and lights for the city below. I made a shape for the dome's cross-section and lathed it, preplanning for extra details like the pipes in the center as I made the splines.

The highway system was made with the 'street' splines being lofted along a path that wandered around the city. They met at the dome supports, which was accomplished with the assembly of rather simple geometry and boolean subtracting the highway's passage through them.
After that, in the spirit of low-poly design I put together a simple highway vehicle so that I could have a whole bunch of them at as low as processing price as possible. Once it was made, I instanced it with a bunch of supersprays and used path follow to follow the highway path splines.